This invention relates to single-mode fiber optic couplers that are capable of effecting a relatively uniform coupling of light between fibers over a relatively broad band of wavelengths.
Fused fiber couplers have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Various coupler properties can be improved by inserting the fibers into a capillary tube prior to heating and stretching the fibers, thereby resulting in the formation of an "overclad coupler". To form an overclad coupler, the fibers are inserted into a tube, the tube is evacuated, and its midregion is heated and collapsed onto the fibers. The central portion of the midregion is thereafter drawn down to that diameter and coupling length which is necessary to obtain the desired coupling.
Identical optical fibers were heretofore used to make a standard coupler, the couplinq ratio of which is very wavelength dependent, i.e. if it exhibits 3 dB coupling at 1310 nm it cannot function as a 3 dB coupler at 1550 nm because of that wavelength dependence. A "standard coupler" might be characterized in terms of its power transfer characteristics in a window centered about 1310 nm, which is referred to as the rust window. For example, a standard coupler might exhibit a coupling ratio that does not vary more than about .+-.5% within a 60 nm window.
An "achromatic coupler" is one wherein the coupling ratio is less sensitive to wavelenqth than it is for a standard coupler. There is no widely accepted definition of an "achromatic coupler". The least stringent definition would merely require an achromatic coupler to exhibit better power transfer characteristics than the standard coupler in the first window. More realistically, the specification is tightened by requiring an achromatic coupler to perform much better than the standard coupler in that first window, or to require it to exhibit low power transfer slopes in two windows of specified widths. These windows might be specified, for example, as being 100 nm wide and centered around about 1310 nm and 1530 nm. These windows need not have the same width; their widths could be 80 nm and 60 nm, for example. An optimally performing achromatic coupler would be capable of exhibiting low values of coupled power slope over essentially the entire single-mode operating region. For silica-based optical fibers this operating region might be specified as being between 1260 nm and 1580 nm, for example.
In the following discussion, the relative refractive index difference .DELTA..sub.a-b between two materials with refractive indices n.sub.a and n.sub.b is defined as EQU .DELTA..sub.a-b =(n.sub.a.sup.2 -n.sub.b.sup.2)/(2n.sub.a.sup.2) (1)
For simplicity of expression, .DELTA. is often expressed in percent, i.e. one hundred times .DELTA..
Heretofore, achromatic couplers were formed by employing fibers having different propagation constants for the fundamental mode in the coupling region, i.e. by using fibers of different diameter and/or fibers of different refractive index profile or by tapering or etching one of two identical fibers more than the other.
U.S. Pat. Nos. 5,011,251 and 5,044,716 teach overclad achromatic fiber optic couplers wherein the coupled fibers are surrounded by matrix glass having a refractive index n.sub.3 that is lower than that of the fiber cladding material. The propagation constants of the coupler fibers are different since the fibers have different cladding refractive indices. The difference between the refractive index n.sub.2 of the cladding of the first fiber and the refractive index n.sub.2 ' of the cladding of the second fiber is such that the coupler exhibits very little change in coupling ratio with wavelength over a relatively wide band of wavelengths.
U.S. Pat. Nos. 5,011,251 and 5,044,716 characterize the tube refractive index n.sub.3 by the symbol .DELTA..sub.2-3, the value of which is obtained by substituting n.sub.2 and n.sub.3 for n.sub.a and n.sub.b in equation (1). Commercially available single-mode optical fibers usually have a value of n.sub.2 that is equal to or near that of silica. If silica is employed as the base glass for the tube, a dopant is added thereto for the purpose of decreasing the tube refractive index n.sub.3 to a value .Iadd.lower .Iaddend.than n.sub.2. Those patents state that in addition to lowering the refractive index of the tube, the dopant B.sub.2 O.sub.3 also advantageously lowers the softening point temperature thereof to a value lower than that of the fibers. Fluorine has also been employed to lower the tube refractive index. Those patents teach that when .DELTA..sub.2-3 is below about 0.2%, the amount of B.sub.2 O.sub.3 in a silica tube is insufficient to soften the tube glass in a 1.times.2 or a 2.times.2 coupler, whereby it excessively deforms the fibers during the collapse step. The value of .DELTA..sub.2-3 for standard couplers has therefore usually been between 0.26% and 0.35%. Those patents further state that process reproducibility of achromatic overclad couplers is enhanced by employing tubes having refractive indices such that .DELTA..sub.2-3 values are above that previously employed range, preferred values of .DELTA..sub.2-3 being greater than 0.4%.